Noise-cancelling headphones are well known. Some noise-cancelling headphones work so well that they can impair a person's awareness of his surroundings either by effectively cancelling certain types of sounds or by providing audio content the output level of which essentially overpowers or “masks” sounds from a person's surroundings.
Similarly, the audio systems provided in many vehicles are so powerful that they too can overpower or “drown out” sounds from a person's surroundings, including for example, sounds of an approaching train or the horn of an approaching vehicle or emergency vehicle siren.
An apparatus and method by which a person can be made aware of their surroundings by recognizing certain types of sounds and providing appropriate warnings would be an improvement over the prior art.